One of the distinctive trends in computer systems is that memory capacities are becoming larger and larger due to the extending of computer networks, popularity of engineering work stations, utilization of data bases and the like. Further, the most common magnetic disc driving apparatus built in computer systems as a memory apparatus has been changed from the 5.25-inch disc drive to the 3.5-inch disc drive, which proves the demand for memory apparatus to be made more compact and slim in size. The demands of magnetic disc driving apparatus, such as the demands for larger capacity, smaller and slimmer size, naturally lead to demands for a spindle motor (hereinafter called simply a "motor") mounted to the disc driving apparatus to be of higher accuracy and smaller size. The higher accuracy, among others, is strongly demanded.
Along with the technology advancement, a memory capacity of the magnetic disc has increased, and the track density of discs can be 8000 TPI (tracks per inch)--10000 TPI, which is converted to a track pitch as fine as 3 .mu.m. The performance required of the motor mounted to the apparatus is to always accurately trace each track having such fine track pitch. This kind of motor has employed ball bearings in general; however, the rotation of ball bearings inevitably generates vibration. The level of vibration is measured to be as fine as ca. 0.15 .mu.m based on NRRO (Non Repeatable Run Out), which is non repeatable sway of the hub of the motor. This vibration level is the minimum possible value for the ball bearings. When this vibration occurs, a magnetic head deviates from a track by the displacement component due to the vibration. This deviation has a harmful influence on read/write operation, and the conventional apparatus employing the ball bearings thus allows almost no margin to meet the required performance.
Recently, a motor has been proposed in order to improve the accuracy, lower the noise level, and extend the product life. The motor comprises a fixed shaft, a sleeve that is supported and rotated by the shaft and a radial-dynamic-pressure-fluid bearing, or the motor comprises a fixed sleeve, a rotating shaft that is supported and rotated by the sleeve and the radial-dynamic-pressure-fluid bearing.
The motor employing the dynamic-pressure-fluid bearing is disclosed in Japanese Patent Application unexamined publication No. H06-178489.
FIG. 16 is a cross sectional view of this conventional motor. In FIG. 16, a shaft 501 is vertically fixed at the center of a bracket 504, and a stator core 510 with wires wound thereon is mounted to the bracket 504. A rotor magnet 506 is fixed to a rotor frame 505 so that the rotor magnet faces the stator core 510. The rotor frame 505 is mounted to the hub 503. A bushing 511 is fixed at a lower section of an inner rim of the hub 503, and another bushing 512 is mounted to an outer rim of the bracket 504. The bushing 511 faces the bushing 512 with a clearance in-between. The magnetic discs (not shown) are to be mounted around the hub 503.
Grooves (not shown) are provided inside of a sleeve 502, the grooves produce dynamic pressure of lubricating fluid by the rotation of the sleeve 502, which is rotatively supported by the fixed shaft 501 via lubricating fluid. Radial-dynamic-pressure-fluid bearings R501 and R502 are thus constructed. Axial dynamic pressure bearings A501 and A502 comprise both end faces of a fixed thrust ring 507, a lower face of rotation thrust ring 508 and an upper face of the sleeve 502. A groove 541 is provided on an outer circumference of a cap 509, and another groove 542 is provided on an inner circumference of the rotation thrust ring 508. The lower rim of groove 541 is disposed at substantially the center of groove 542, and the upper rim of groove 542 is disposed at substantially the center of groove 541. The upper and lower rims of each groove 541 and 542 face each other with some offset.
The conventional motor employing the above dynamic-pressure-fluid bearing has a possible problem that the lubricating fluid might splash into a space where the magnetic discs are disposed. In this space, a magnetic head reads/writes data from/to the magnetic disc with little clearance between the head and disc. The space thus must be kept utmost clean because if the lubricating fluid splashes or flows into the space, serious problems such as a head crush, a head absorption, etc. will occur. (Hereinafter the above space is called the "clean space".)
The above conventional motor has provided a countermeasure against lubricating oil splashes by forming an oil pool using the grooves 541 and 542 to prevent the lubricating fluid from splashing out from the upper part of the motor; however, this countermeasure cannot prevent a mist of lubricating fluid from flowing out.